Low-energy,low-temperature mass spectra. 10—urethanes

The 12.1 eV, 75°C electron impact mass spectra of 24 urethanes, RNHCO₂C₂H₅ R ? H, C₂H_{2n +1} (n = 1-8), CH₂?CHCH₂, Ph, PhCH₂ and PhCH₂CH₂], and seven symmetrically disubstituted urethanes R₂NCO₂C₂H₅ (R ? C_n H_{2n + 1} (n = 1?4)) are reported and discussed. All 31 spectra show appreciable molecular ion peaks. For n ?Cn H_{2n +1} NHCO₂C₂H₅, M^{+ ˙} usually is the most abundant ion in the spectrum. A peak at m/z 102 of comparable intensity also is present; this corresponds to formal cleavage of the bond connecting the α- and β-carbon atoms in the N-alkyl group, though it is unlikely that the daughter ion has the structure CH₂?NHCO₂C₂H₅]⁺. In the RNHCO₂C₂H₅ series, branching at the α-carbon atom enhances the relative abundance of the ion arising by notional α-cleavage at the expense of that of M^{+ ˙}. Formal cleavage of the bond between β- and γ-carbon atoms occurs to some extent for RNHCO₂C₂H₅]⁺˙ ions; this reaction provides information on the degree of branching at the β-carbon, especially if metastable molecular ions are considered. The higher n-C_nH_{2n +1}NHCO₂C₂H₅ (n = 5?8) urethanes exhibit two other significant ions in their mass spectra. First, there is a peak at M ? C₂H₅]⁺. Secondly, a peak is present at m/z 90; the most plausible structure for this ion is H₂N(HO)COC₂H₅]⁺, arising by double hydrogen transfer from the alkyl group and expulsion of a C_nH_{2n ?1}]˙ radical. Ions originating from secondary decomposition of the primary ionic species are generally of only very low abundance in these spectra.